dm cache: delete the redundant word 'each' in comment

Signed-off-by: Shaomin Deng <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-cache-policy.h

···
 	struct dm_cache_policy_type *real;
 
 	/*
-	 * Policies may store a hint for each each cache block.
+	 * Policies may store a hint for each cache block.
 	 * Currently the size of this hint must be 0 or 4 bytes but we
 	 * expect to relax this in future.
 	 */